How many kingdoms are there in the domain Archaea?

In the domain Archaea, there are currently four recognized kingdoms based on modern classification systems, although this field is still evolving. These kingdoms are:

1. Euryarchaeota: Includes methanogens (methane-producing organisms), halophiles (salt-loving organisms), and some thermophiles (heat-loving organisms).
2. Crenarchaeota: Primarily composed of thermophiles and acidophiles that thrive in extreme temperatures and acidic environments.
3. Thaumarchaeota: Contains ammonia-oxidizing archaea, which play a key role in the nitrogen cycle and are found in both extreme and moderate environments.
4. Korarchaeota: A lesser-known group found in extreme environments, thought to represent some of the earliest diverging archaea.

Some scientists propose additional groups, but the classification of Archaea is an active area of research, with new kingdoms possibly emerging as discoveries progress.